I beg to differ the song is in A dorian mode the v chord ,E is the tonic making it a V-IV-I progression E is the relative minor of G major and A dorian is the second mode G major allowing the use of the E minor blues scale since the F# is omitted from both A dorian and E minor pentonic the only passing tone I have found in the solo is Bb which is the blue note in E minor blues scale

There's a difference between a key signature and a key. You could make a case for the key of E minor, which shares a key signature with the key of G, but there's absolutely nothing about the song that would indicate it's in the key of G major.

Actually this piece is in the key of d Maj relative b minor. It appears that the opening power chord of e is a minor chord. Iv-v-i progression. What angus has done here has switched the iv chprd gmaj for its relative e minor.A brilliant mive on his part .

Actually this piece is in the key of d Maj relative b minor. It appears that the opening power chord of e is a minor chord. Iv-v-i progression. What angus has done here has switched the iv chprd gmaj for its relative e minor.A brilliant mive on his part .

It moves from E5 to D5 to A5. If it were in the key of D, that's a ii-I-V progression, not a vi-V-I.

The term "key" is a bit loose in music, so there are several interpretations.

You can make a case for a song being in the "key" of the "key signature", the set of sharps or flats that generally define the pitches used. You can make a case for "key" being the set of chords being used. But the best definition of "key" is the identification of the tonic, the note that serves as home base for the progression.

In Back in Black, almost every phrase (and the song as a whole) ends on an A. That's the point of resolution. So it's in the key of "A something".

Throughout the tune we also have B5 and G5 power chords. B is the ii of A, but G isn't in the key of A major - it would be the flatted seventh. So if you're trying to frame the tune within a single key, it's in the key that has A as the tonic, but a b7. And that's A Mixolydian.

A Mixolydian has the same notes as the key of D major, but it isn't the same as D major - because D is not the tonic. No phrases end on a D, and every time you hear the D5 you have a sense that it the progression hasn't ended yet.

Yes, it does. And it doesn't feel like the song is over at that point, because the D leaves you hanging. You get that feeling because D is not the tonic (which means it isn't the "key"). The song ends on the album by fading out, but if you end it with the main riff followed by an A power chord it WILL feel like the ending, because that's the tonic.